The fuel cell is capable of producing electricity without a substantial need for fossil fuel that poses concerns about resource depletion, without noise, and at a high energy recovery rate as compared with other energy-based power generating systems. Great efforts have been made to exploit the fuel cell as a power generating plant of relatively compact size in buildings and factories, with some cells having been commercially implemented. In particular, polymer electrolyte fuel cells (PEFC) can operate at lower temperature than fuel cells of other types. The PEFC then draws attention not only as a device for household co-generation, but also as the replacement power source for internal combustion engines on vehicles because of the minimized corrosion concern regarding the materials of which cell components are made and their ability to discharge relatively high current flow despite low temperature operation. The PEFC is constructed of electrolyte membranes, separators and other components. The separator is generally a plate which is provided with a plurality of parallel channels on one surface or both surfaces. The separator plays the role of conducting the electricity produced at the gas diffusion electrode within the fuel cell to the exterior, discharging water produced within the channels in the course of electricity generation, and securing the channels as a flow path for incoming reaction gas to the fuel cell. Such a fuel cell separator is required to be more compact in size. Since a multiplicity of separators are used in stack, there is a demand for a separator seal material having durability and long term service.
As the separator sealing material, packing materials based on various resins have been under study. Among them, sealing materials based on silicone rubber are often used for their moldability, heat resistance and elasticity. JP-A 11-129396 and JP-A 11-309747 disclose silicone rubber compositions of the addition cure type featuring easier molding than conventional silicone rubbers. Silicone rubbers obtained by curing these compositions have been used, but are still unsatisfactory in maintaining elasticity over a long term. In particular, for improving the seal performance in acidic aqueous solution which is requisite as the packing material for fuel cell separators, it was necessary to minimize the amount of reinforcing fillers such as fumed silica. Regrettably, reducing the amount of reinforcing fillers gives rise to several problems such as reduced strength of cured rubber, difficulty of molding, and failure of seals. These problems can be solved using a mixture of a silicone fluid having alkenyl groups at ends of the molecular chain and a silicone fluid having alkenyl groups on side chains as disclosed in JP-A 2003-257455. For the sealing material for PEFC separators, not only rubber strength and low compression set, but also adhesion to the separator substrate are important factors. JP-A 2004-14150 describes a silicone rubber having minimal compression set in long-life coolant (LLC) as an exemplary gasket material for carbon separators, but refers nowhere to the adhesion of the silicone rubber. JP-A 2007-146147 describes a primer to be applied prior to integral molding of silicone rubber to a substrate to form a fuel cell separator. The rubber material is only described therein as being of the addition cure type, with no reference to its detail.